FIG. 1A shows a remotely powered contactless card 1 arranged in the vicinity of a terminal 3. Terminal 3 comprises an antenna 5 and an electronic circuit 7. Contactless card 1 comprises an antenna 9 connected to a data processing unit 11, via rectifier 13. Unit 11 and the rectifier are connected to a ground of the card.
The terminal permanently emits a field. Thus, when the card is close to the terminal, it is powered by antenna 9 and is capable of receiving a signal 15 and of transmitting a signal 17.
FIG. 1B shows, as a function of time, signal 15 transmitted by antenna 5 of the terminal. FIG. 1C shows signal 17 transmitted by antenna 9 of the card. Four phases can be distinguished.
During an initial phase P0, the card receives from the terminal power which enables to activate processing unit 11. The presence of the card is sensed by the terminal. The terminal then combines, during a phase P1 of reception by the card, a data signal to the power supply field that it emits, and the data are received by antenna 9 of the card and transmitted to unit 11. Unit 11 then processes the data during a data processing phase P2 to prepare the response. The response is sent by the card to the terminal during a phase P3.
To send the response, unit 11 varies current I that it consumes. Current variations 23 in antenna 9 are then detected by the terminal.
A problem is that, during data processing phase P2, current I consumed by unit 11 varies over time, due to the data processing. This causes a variation 25 of the current in antenna 9. Such a variation may be incorrectly interpreted by the terminal as a response of the card, which would result in an error.